This invention relates to the deposition of metals on metallic substrates, and, more particularly, to the deposition on substrates that reform oxides rapidly.
Molybdenum is widely used in the microelectronics industry in holders for ceramic packages. The packages, in turn, receive and support microelectronic devices such as integrated circuits. Molybdenum is used in this application because its coefficient of thermal expansion closely approximates that of typical ceramics used as packages. When the holder, package, and integrated circuit are heated or cooled, there is little tendency for the molybdenum to debond from the ceramic.
The ceramic package is typically soldered or brazed to the molybdenum holder. The molybdenum is first coated with copper, prior to the joining operation, to facilitate joining. The deposition of a layer of copper on molybdenum is difficult, because molybdenum quickly forms a tenacious oxide after cleaning operations. If the molybdenum oxide on a molybdenum surface is removed, as by etching, the oxide layer reforms in a time on the order of milliseconds. The reformed layer is initially very thin. It is, however, sufficiently thick to prevent ready adherence of copper or a nickel strike layer to the molybdenum surface, even if the deposition is accomplished immediately after etching. The deposition of a copper layer on molybdenum is therefore not readily performed, and requires a special process.
Several techniques have been developed to permit deposition of a copper layer upon molybdenum. The chromium deplate/plate process first removes the oxide by making the molybdenum the anode of a cell. The molybdenum is then moved to the cathode, and chromium is deposited in an effort to prevent the oxide from reforming. A nickel strike layer is deposited over the chromium, and a copper layer is deposited over the nickel layer. The process is inefficient and largely ineffective, because the oxide can reform within milliseconds and because the eletrochemical plating efficiency of the chromium is only about 15%. Typically, only 60% of the plated parts produced by this method are acceptable. The time required to shift the molybdenum piece from the anode to the cathode permits the oxide to reform in an amount sufficient to interfere with the chromium plating. Even where the process is operable, the resulting layer of chromium and nickel is magnetic, and may adversely affect the operation of the microelectronic device mounted on the ceramic. Finally, but perhaps most significantly, the process requires the use of hexavalent chromium, which can be an environmental hazard when it is disposed of.
A second approach utilizes a hydrogen furnace treatment of the molybdenum piece. The molybdenum surface is etched to remove the oxide, and then cycled in a hydrogen furnace. Immediately after this step, a gold strike layer is deposited upon the surface. The molybdenum piece is returned to the hydrogen furnace, and the process of depositing gold and refiring in hydrogen is repeated three additional times. Finally, copper is deposited overlying the gold strike layers. This process is successful in providing a copper layer, but is expensive, as a hydrogen furnace costs several hundred thousand dollars, and gold plating is used. The process is also time consuming, since about 16-20 hours is required to complete the deposition cycle.
Other, more exotic, techniques have been used to deposit copper onto molybdenum. In magnetron sputtering, for example, a gold strike layer is deposited upon the molybdenum by sputtering in a vacuum, and then the copper layer is deposited overlying the gold layer. The principal difficulty with such approaches is that they are expensive and not adapted to mass production of irregularly shaped molybdenum parts on a routine basis.
At the present time, there is no mass production technique for depositing a copper layer on molybdenum, that is efficient in producing a high percentage of good pieces, and is also inexpensive. Such a process is needed, particularly in the electronics industry and in other areas where molybdenum is used in similar applications. There is also a need for a process for plating a copper layer onto molybdenum without introducing magnetic material which can be detrimental for certain applications. The present invention fulfills these needs, and further provides related advantages.